The FCC, or fluidized catalytic cracking process, is a mature process. It is used to convert relatively heavy, usually distillable, feeds to more valuable lighter products. There is an increasing need in modern refineries to convert more of the "bottom of the barrel" to more valuable lighter products, e.g., resids or residual oil fractions.
In the past these heavy streams were subjected to various thermal processes such as coking or visbreaking to convert them to more valuable products. Unfortunately, thermal processing alone has not proved to be a complete answer to the problem, as the products of thermal cracking are themselves relatively low valued products, such as heavy fuel oil from visbreaking or coker naptha or coker gas oil from coking operations. In the case of coking, very large coke yields result in large volumes of low value product.
Residual oils have a large percentage of refractory components such as polycyclic aromatics which are difficult to crack. Resids also contain large amounts of metals which rapidly deactivate conventional catalyst. Some attempts at catalytic processing of these stocks have been made e.g., adding relatively small amounts of residual oil to conventional FCC feed. FCC units can tolerate modest amounts of resids in the feed, e.g., 5-10 wt percent but the heavy feeds increase the burning load on the regenerator (because of their high Conradson carbon content) and poison the catalyst, with nickel and vanadium. Limiting the amount of resid in the FCC feed has been the method of choice in controlling regeneration operation, although consideration has been given to adding catalyst coolers. The nickel and vanadium contamination problem can be overcome to some extent by practicing metals passivation, e.g., addition of antimony to the unit to passivate the metals added with the feed. Metals passivation has allowed FCC units to continue operating with catalyst containing relatively high amounts of nickel and vanadium, but has not been a complete solution. The vanadium seems to attack the zeolite structure of modern FCC catalyst, resulting in rapid loss of catalyst activity. The exact cause of vanadium poisoning is not completely understood, but it is believed that oxidized vanadium compounds are formed in the highly oxidizing atmosphere of conventional FCC regenerators and these compounds, particularly vanadic acid rapidly attack the zeolite. The problem is discussed in Vanadium Poisoning of Cracking Catalyst, Wormsbecher et al., Journal of Catalysis, 100, 130-137 (1986).
Most refiners now monitor the metals concentration on their catalyst and dump equilibrium catalyst and replace it with fresh catalyst to control the average level of metal on the catalyst. Such a solution is expensive because it can result in very high catalyst replacement rates.
Another approach to adding residual oils to FCC units is described in U.S. Pat. No. 3,886,060, which is incorporated herein by reference. Residual oil was used as a quench medium to limit the conversion of a recycle oil in a riser conversion zone. The preferred catalysts were dual components, i.e., containing both large and small pore size zeolites. A single regenerator operated with dual riser reactors.
Despite the many improvements which have been made, attempts to crack resids have not been too successful, primarily because of the large amounts of metal and coke associated with such feeds. We have now discovered a way to handle such difficult stocks in a single riser reactor, using a two stage regenerator. In this approach, we use a mixture of coarse and conventional catalyst in a single riser reactor with a two stage regenerator to achieve some unusual results.
By careful selection of the catalyst sizes and of the superficial vapor velocities in the catalyst regenerator it is possible to keep the coarse catalyst effectively segregated from the conventional catalyst. The coarse catalyst is regenerated in a single stage, under relatively mild conditions which minimize oxidation of vanadium compounds on the catalyst but which still remove much of the hydrogen content of the coke and eliminate most of the water precursors. The conventional FCC catalyst is regenerated to some extent in the first stage regenerator, and then undergoes a second stage of regeneration, at a higher temperature, with higher oxygen concentrations. Use of two different kinds of catalyst, in a two stage regenerator, permits significantly higher metals levels to be tolerated in the feed.